Harmonic vibrations in Jeeps: A new theory (please read!)

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TJ Addict

A lot of us have vibration issues of some sort in our Jeeps. Vibration is primarily of two kinds, harmonic (i.e rhythmic, pulsating with a specific frequency) or non-harmonic (the various rattles and squeaks and non-rhythmic). There is a also "wobble", for example one coming from improperly balanced wheels (yes, that's a vibration you feel at specific speeds).

In this thread, I am going to be talking about harmonic vibrations. In our jeeps, there are two kinds of these. The common one that lot of people have experienced is driveline vibrations from improperly set pinion angles. Watch this video to understand why they happen and why precise setting of the angles and proper balance of the driveshafts are both very important.

The second kind, and the one that has tested the patience of many, many Jeep owners is the harmonic vibration in the tub that happens after re-gearing the axles to a different gear ratio. They check tire balance, driveshaft balance, pinion angles and everything is in order but the harmonic vibration is still persistent after a specific speed.My jeep currently has this problem after a recent regear at one of the best local so shops installed by a Jeep expert. I felt a gradual onset of vibrations and just past 60mph, it felt like the entire Jeep started to vibrate. I felt it in the pedals, steering wheel, seats, console armrest and heard a unpleasant droning sound. Since then, I have been doing a LOT of reading and thinking about this. I have a background in physics and I am approaching this problem from that perspective.

The most important thing to know about harmonic vibration is that all materials have a natural resonant frequency of vibration. Once they start moving near this frequency, vibrations will increase and at the precise natural resonant frequency, the vibration amplitude (ie the amount it moves) will be maximum. Think of guitar strings; all strings are the same length. Each string is of a different thickness (and therefore, mass) and they have a characteristic sound depending on that thickness/mass and length. There is a fundamental (or base) frequency of oscillation and what are called "higher order" oscillations that are numeric multiples of this fundamental frequency (that are higher in pitch compared to the base frequency).

Same applies to parts in our jeeps, like the frame. Yes, the frame will have a frequency and it is fixed. So will the transfer case. So will the skid plate and so on and so forth. Car manufacturers go to great lengths to make the critical engine/trans parts very precisely sized to get the performance and clearance they need. In addition, all engines have a harmonic balancer fitted to the free end of the crankshaft to counteract the resonance vibrations from the moving parts. Note that these are VERY precisely designed, made of a specific shape, size and material depending on what vibrations they are made to reduce.

Our Jeep Transfer cases also come with such harmonic balancers. Both the 231 TC and the 241 TC have these. Below are pics to show them visually (top two are 241 , last one is 231). Removing this balancer while installing a SYE or aftermarket driveshaft could be very risky thing to do, in my opinion. The factory went to great lengths to put it there and we risk vibrations without this component. This is a really important to think about. Better driveline angles and stronger driveshafts are important, but equally important is this balancer and no one seems to think about removing it!

In the last 4 weeks of reading an enormous number of vibration threads post regear on different forums, I have come across just two cases of people solving their vibration issues. One was a guy who found out that his driveshaft u-joints were installed out of phase (easy fix!) and the other guy's shop removed his harmonic balancer and the problem went away. That's it, just two cases out of many, many people who are just living with vibrations having tried whatever they could with driveshafts and wheels and whatever else they could think of.

You might be asking me how the vibrations went away after removing the balancer in that guy's case, or that why you don't have a problem in your jeep with or without the balancer. I will try to explain how. I believe that this is not a magic, unexplainable problem but that each Jeep being different, they each have a variety of things happening at the same time making the problem difficult to communicate and comprehend.

Note that the problem is indeed nuanced. First thing, the gear ratio matters since going to deeper gears you tend to see this problem sooner. Second thing, you may have other aftermarket mods that may be actually helping you. With deeper gears, now the driveshaft is moving at a frequency that is closer to the natural resonant frequency of the system. In our jeeps (specifcally 03-06), the TC skid has a resonant frequency that is already close to the normal driveline vibrations. So, the faster the driveshaft is spinning (with deeper gears), the more prone you are, to setting this resonant vibration in the system which you hear and feel inside the tub. See the comment from Jim Frens (prev owner of Nth degree and ex-Jeep engineer, very knowledgeable guy) on the topic of vibrations in jeeps on a different forum (emphasis mine):
"Vibration problems in ‘03+ Wranglers are mostly due to poor decisions that were made at Jeep several years back (when engineering the ‘03’s started). The stock center skid for ‘03+ TJ’s has a natural frequency that lines up with the normal powertrain vibrations, and so acts as a ‘speaker’ to make this energy very apparent in spite of the rubber isolators. The other problem is that your new 42RLE automatic is derived from the FWD transaxle in the Dodge Stratus/Cirrus cars, and unfortunately has their carryover gear ratios (way too wide with a horrifically tall OD ratio of .69:1). And guess what? The OD ratio sets up a harmonic with the third-order firing frequency of the I-6 engine because they are only 2% apart, so you have a powertrain that just loves to ‘sing’ – and the only part of it you can do anything about is the center skid, which is why it’s been learned that changing out the center skid does help a bit (removing the speaker by changing it’s natural frequency, since the natural frequency of the beefier skid is much higher – this applies to both the RELA skid and our Tummy Tucker). Vibration problems in a lifted vehicle are only about 25% dependent on the balance of the driveshaft itself and it’s operating angles, the rest is natural frequencies, etc. – unfortunately virtually none of the end-users w/ lifted TJ’s know anything about this and so get mad at whatever they think is the culprit – and in fact no one thing usually is – Jeep did some things that didn’t help, and the lifting certainly didn’t help, and incorrect set-up of the lifted suspension only adds to this mess, so there’s much more than meets the ‘eye’ (or in this case your *** feeling the vibes!)."

So how do you fix this? As Jim Frens says, You have to change the mass of the resonant system (since the frequency depends on the mass). You can perhaps do it by going a thicker, heavier skid as mentioned above (without installing a body lift). Note that Dave kishpaugh told me that he has dealt with this problem in the past on some stubborn jeeps by installing dampening material on the inside of the skid plate.

There is also another way. You can mess with the harmonic balancer, which is designed for a specific speed range; either take it away, reducing the previously correctly tuned weight, or by installing a heavier balancer of the right shape and size. In the case of the jeep owner who's vibrations went away, this is precisely what happened. They accidentally removed the balancer and tried it and found out that the vibrations were gone. He never investigated further as to why it went away after his saga, but he does mention it here. To my knowledge, no one has ever tried installing a different balancer. This component is easily made once you know what size and weight you need. One other simple way could be to add a bunch of wheel weights (the stick on kind) all around the periphery and see what happens.

Now, to further strengthen my arguement, here is a post from the very knowledgeable MrBlaine on a different forum (emphasis mine)
"The Rubicon 4-1 T-case does have the option of having a conventional yoke installed in place of the flange mount on the face of the harmonic balancer.

That is done typically for either when the conversion to a Double Cardan style driveshaft is installed and if the yoke is present, it will resemble the conventional SYE on other TJ t-cases since the Rubi case already has the SYE from the factory.

Some folks install the yoke style under the mistaken impression that they are better or that they will wind up with a longer rear driveshaft and neither is true. The 241 has an inherent need for the balancer most especially when certain gear ratios are in play. It is fairly common for those with the 241, 5:13's and the auto to have high levels of difficulty dialing out vibes that cyclical in nature.

Some Rubis I've messed with are obstinately particular about the driveshaft angles and the only way to know if they are correct is to make minor changes starting with the front one removed and keep dialing it up or down until you get it vibe free and then install the front shaft and do it all again.

The rear driveshaft is more forgiving when the angle is lower, the front is more forgiving when the angle is higher. Neither can be ascertained when you get one that is a PITA simply by looking at it and my helper and I set driveshaft angle at least once a week simply by looking at them and we are rarely wrong except when it comes to that small handful that are problems. When we get one of those, we break out the angle finder and we've found a few where the rear driveshaft angle was critical to be adjusted to within 1/2° which is not able to be done by eye"

You can see the flange mount on face of the harmonic balancer in the first couple photos I posted. I wonder if that is part of the harmonic balancer "system" in terms of weight. In my 05 TJR, I had gotten a double cardan driveshaft installed to get rid of the TC drop that the previous owner installed when he lifted the vehicle. I do not know if I have the flange mount currently (guessing not, and perhaps that changed the weight balance).

The reason I'm going to this much detail is to show how these changes all could interact and why one Jeep has a problem and another doesn't, although the Jeeps outwardly have same trans, axle gear ratio and lift. The devil is really in the details, and how your Jeep is set up.

Now, also note that you could be having driveshaft vibrations (recent lift, pinion angles etc) and/or wheel balance induced vibrations, on top of the resonant harmonic vibrations and it'll be very hard to decouple them. So everything needs to be checked and re-checked as Blaine says above.

I am going to try and have Dave check the harmonic balancer idea on my jeep. I am confident we will find a solution. I really think this is what is happening in our Jeeps w.r.t harmonic vibrations after a regear. It took me a while to assimilate and come up with this consolidated thought since information is pretty scattered on the forum threads.

I have been talking to @Chris a lot and he may have more thing to add that I may have missed. I invite your comments and discussion on this topic.

NEW FINDINGS 11/2/2018

The harmonic dampener is added to help curb 55Hz frequencies (as told by the production engineer himself on another forum).

From reading the FSMs for all the years, this is what I gathered:

1997,1988,1999, 2000 jeeps - no balancer in autos or manual trans (they all had 231 TC)

2001-2002 - damper only on AX5/NV3550 manual trans only, auto trans did not get it (they all had 231 TC).

All 2003-2006 jeeps have the same TC skid (Rubicon and non-Rubicon models)

Forum members have confirmed that 32RH jeeps in 2000/2001/2002 do NOT have the balancer.

FSM only mentions 4.0L models in any of the years they put the balancer on. 2.5Ls are not mentioned in any of the years.

Given this info, I postulate that there are 4 variables to be used in figuring out if your jeep has a balancer (and is therefore prone to resonant harmonic vibration). The combo of Year+engine+TC+trans+axle gearing tells you the information. I will post a good table inside the thread later. @Chris is helping me with it.

TJ Addict

Tagging @glwood and @zebra12 who are potentially looking to regear their jeeps.

Also paging @jjvw and @bobthetj03 since you guys are knowledgeable and like discussing theory And of course the very knowledgeable @Jerry Bransford to see if he has anything to say on this topic and my line of thinking.

Also paging @Mr. Bills since this is relevant to the issue in his Jeep. Please read through this thread. You could try the fix @Chris and I are putting forward (changing weight of the existing damper) if you are interested!

I apologise if my post above reads like an undergrad trying to explain his newfangled (and potentially wrong) addition to the theory of relativity :risas3:

Administrator

In my mind, this has to be the cause of the harmonic vibrations people experience. I'm not talking about the more violent vibrations you can feel in the steering wheel, pedals, and seat. Those vibrations have to be related to driveshaft, driveshaft angle (even Blaine has said that he's found some TJs to be very, very, very sensitive to driveline angles as oppose to others), or even tire imbalance issues.

I'm talking about the harmonic vibrations that could be compared to a sine wave, or the vibration you'd hear after hitting to small bells together.

Why you you think that a harmonic balancer is put on the transfer case output shafts or the crankshaft for that matter? Those harmonic balancers aren't just some random weight or size either. They're a specific weight and a specific size that has been figured out by the engineers. I'm guessing that that size and weight goes along with the gearing of the vehicle and how fast they expect these components to be spinning.

When we gear deeper and therefore spin things at different speeds, the harmonic balancers are no longer doing their job 100%.

So why are some TJs experiencing this while others aren't? My thought has to do with the amount of noise TJs produce in general. Most of us have loud exhausts, no sound dampening material like a modern vehicle, lots of rattles, stiff rides, etc. All of this stuff probably masks the harmonic vibrations that may be more prevalent in some TJs, and less prevalent in others.

How could we test this out? Well, we could go about machining new harmonic balancers with more weight (it wouldn't be that difficult of a part to machine... actually, you might just be able to machine a ring that would attache to the existing one). However, I think a good way to test this would be to get some of those double sided wheel weights and place them evenly across the edge of the harmonic balancer on the transfer case.

It would be some sort of random weight (though all the individual weights would be equal in weight), but it would be interesting to see if adding that weight changed any perceivable difference in the harmonic vibrations coming from it.

I'm no engineer, so I'm only speculating. However, I'm pretty certain @psrivats is onto something here.

While he's also not an automotive engineer, the guy does have his PHD and designs microchips for Intel, so I can assure you he's got the brains!

TJ Addict

To just clarify and add to @Chris' post above before someone unintentionally misinterprets what he's saying, I'll add that the fundamental harmonic vibrations can be violent (the entire structure is vibrating after all .. an earthquake is a vibration too!) and if strong enough, will be felt in the seat and pedals, mirrors, everywhere. However, it will pulsate. I'm sure some of you guys have walked on bridges and felt the whole bridge moving in the wind - that's a resonant harmonic vibration, but you'll have noticed that it's not constant but pulsates.

Driveshaft vibrations will also be felt in the same places, but may or may not be constant and these will be additive if present together. If you have both, there is no way to tell them apart and you'll likely hear a constant drone and feel the vibrations. So that's why the first instruction on the forums typically is to check the wheels and the driveshaft angles/ balance.

0-60 in 18 seconds

I don't mention it very often because I don't go this fast on a regular basis, but sometimes mine just begins to develop a cyclical vibe right around 75mph. Sometimes, but not always. I have suspected it is driveshaft related since the 4" lift went in and the way my pinions are set to accommodate the suspension travel. I want to dig into it more someday. But it is low priority right now.

TJ Addict

We sent a message to Dave and this what he had to say. Perhaps I'm on the right track. I've requested Dave to test this theory out further in my Jeep.

I'll continue reading and when I find something more useful to add, I'll post here.

Dave Kishpaugh
"That's one of the theory's I set forth in 2004 when I first started experiencing rubicon issues. I've actually had better luck with eliminating the Rubicon damper on a few. There's some pretty lengthy jeep forum threads between 2004 and 2010 I was involved in and still haven't been able to nail them all. A couple guys have claimed the vibration had disappeared after installing an aftermarket high clearance skid. Pretty sure one was a rockmen 3" and another the savvy ua when it first came out. Rokmen still uses the factory mount. I had an nth degree skid on my 03 auto."

TJ Addict

I don't mention it very often because I don't go this fast on a regular basis, but sometimes mine just begins to develop a cyclical vibe right around 75mph. Sometimes, but not always. I have suspected it is driveshaft related since the 4" lift went in and the way my pinions are set to accommodate the suspension travel. I want to dig into it more someday. But it is low priority right now.

Interesting. I wish my issue were at that speed but alas it's very strong even at 60mph and very unpleasant. Just out of curiosity, what gears do you have in your Jeep @jjvw? Factory or aftermarket driveshaft? Harmonic balancer present or gone?

From my understanding, all harmonic vibrations pulsate (hence the name harmonic, meaning sinusoidal) but if the frequency of oscillation is too high or too low, we'll not perceive it to be pulsating. Just like movies at 24 frames per second, or flourescent bulbs flickering at 60 cycles per second is not visible to the eye, or like in a bowed violin string.

0-60 in 18 seconds

Interesting. I wish my issue were at that speed but alas it's very strong even at 60mph and very unpleasant. Just out of curiosity, what gears do you have in your Jeep @jjvw? Factory or aftermarket driveshaft? Harmonic balancer present or gone?

My setup may be a little unusual. Currently, my rear pinion is set just a bit high of what it maybe ought to be at ride height. The reason is that my 12" rear shock travels just happen to fall within the driveshaft bind at full bump and full droop. The pinion is set to be in the middle of that travel. But the middle of travel is not ride height. A limit strap over the diff would allow the pinion to safely come down a hair and maybe reduce the cyclical vibes at high speeds. It's an up coming project.

Backwoods Amateur

The old one that blew down in the wind storm. The wind matched the the natural frequency of the bridge and set up a torsional vibration in the deck. The energy built up until the bridge self destructed. The video is impressive, and avaliable online along with the engineering analysis of the failure. Required case study for mechanical and civil engineers 40 years ago.

The Navy is hyper interested in this subject as well. Low frequency vibrations are detectable for hundreds of miles on newer SONAR. DARPA probably has reams of paper on the subject. I know we worked on this for days in the engine room on the ships I was on.

TJ Addict

My setup may be a little unusual. Currently, my rear pinion is set just a bit high of what it maybe ought to be at ride height. The reason is that my 12" rear shock travels just happen to fall within the driveshaft bind at full bump and full droop. The pinion is set to be in the middle of that travel. A limit strap over the diff would allow the pinion to safely come down a hair and maybe reduce the cyclical vibes at high speeds. It's an up coming project.

I think the 42RLE is screwed not just from the 0.69 ratio but from it's propensity to induce these harmonic vibrations as Jim Frens notes. I'm starting to read JK forums (they also have the same vibrations issue with 42RLE, looks like) to see if anyone had tried anything on these lines.

TJ Addict

The old one that blew down in the wind storm. The wind matched the the natural frequency of the bridge and set up a torsional vibration in the deck. The energy built up until the bridge self destructed. The video is impressive, and avaliable online along with the engineering analysis of the failure. Required case study for mechanical and civil engineers 40 years ago.

The Navy is hyper interested in this subject as well. Low frequency vibrations are detectable for hundreds of miles on newer SONAR. DARPA probably has reams of paper on the subject. I know we worked on this for days in the engine room on the ships I was on.

TJ Addict

Can someone take good photos of the 231 and 241 balancers, with some input on the dia/thickness if possible? I'm curious to see if they are different in size. That would be a great thing to know since Rubicon came with deeper gears from the factory. Having a hard time guessing at size looking at photos on Google images.

2005 Unlimited Rubicon auto

When I cruise at 60 in third (OD off) I have a harmonic vibe (I hear it, the steering wheel doesn't shake). I am waiting patiently to see what solution presents itself. Your harmonic balancer theory seems to explain why Toyota recommended Tundra driveshafts be set out of phase.

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Produced from 1997-2006, many consider the Jeep Wrangler TJ to be the last true Wrangler. I started Jeep Wrangler TJ Forum to serve as a resource for TJ owners around the world. You'll find everyone here to be friendly, and most importantly, drama free!